Measurements of Sulfur Capacity Proportional to Zinc Sulfidation on

Development of a Regenerable Desulfurization Sorbent for Hot Coal-Derived Gas. Makoto Nunokawa , Makoto Kobayashi , Hiromi Shirai. 2006,383-390 ...
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Ind. Eng. Chem. Res. 2002, 41, 2903-2909

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MATERIALS AND INTERFACES Measurements of Sulfur Capacity Proportional to Zinc Sulfidation on Sorbent Containing Zinc Ferrite-Silica Composite Powder in Pressurized Coal Gas Makoto Kobayashi,* Hiromi Shirai, and Makoto Nunokawa Yokosuka Research Laboratory, Central Research Institute of Electric Power Industry, Nagasaka 2-6-1, Yokosuka 240-0196 Japan

Zinc ferrite-silica composite powder was subjected to a series of reduction and sulfidation tests performed at mainly 450 °C in a simulated coal gas environment. In situ X-ray diffraction analyses revealed that ZnS and FeS were produced at a H2S concentration of 1 vol %, whereas only zinc sulfides were detected at 80 ppm. The scheme for sulfidation at 450 °C was essentially the same as that of pure zinc ferrite at 550 °C. The desulfurization sorbent containing the composite powder was subjected to pressurized thermobalance to determine the sulfur capacities of zinc and iron separately by applying high and low H2S concentrations for sulfidation. This method was applied to analyze the change of the zinc-related sulfur capacity of sorbent samples that experienced multiple desulfurization cycles up to 20 times. At the end of the cycles, the zinc-related sulfur capacity had fallen to about 50% of its initial value, whereas the sulfur capacity due to iron sulfidation maintained 91% of its initial value. This indicates that the residual sulfur mainly results from the production of zinc sulfate during multiple desulfurization cycles. Introduction A regenerable desulfurization sorbent for hot gas cleaning that is able to reduce concentration of sulfur compounds below the part-per-million level requires zinc-containing oxides as a reactant. Zinc ferrite,1-3 zinc titanate,4,5 and other double oxides have been investigated for many years, because these oxides have thermodynamically favorable properties for sulfur reduction. Zinc ferrite-silicon dioxide composite powder6 has been investigated as a candidate material for a high-temperature fuel gas desulfurization sorbent. Sulfur removal with the double oxide was achieved with sulfidation reaction of both zinc and iron. When the sorbent is used in multiple desulfurization cycles, which consist of desulfurization, regeneration, and reduction, the sulfur capacity of the sorbent decreased as a result of sulfate formation.7 Although the tendencies for sulfate formation of zinc and iron might differ under specific reaction conditions, no suitable methodology is available to distinguish the amount of sulfate in terms of the kind of metal. Because the sulfur removal performance at lower concentrations of around the part-per-million level depends on zinc, sulfur capacity of zinc portion is essential to maintain the performance. Thus, it is very important to distinguish the capacity decrease due to zinc sulfate production. It might be effective to reveal reaction schemes of zinc and iron during sulfidation to compare the reactivity of the two metals. Reaction schemes for sulfidation were estimated indirectly from * Corresponding address: 1-6-1 Ohtemachi, Chiyoda-ku, Tokyo 100-8126 Japan. Phone: +81-3-3201-6601. Fax: +813-3287-2841. E-mail: [email protected].

thermodynamic calculations7 and thermogravimetry.8 The estimation was based on the stoichiometry of the sulfidation products: zinc and iron produce sulfides with a definite ratio of metal to sulfur, i.e., ZnS and FeS. This assumption is also used to calculate the sulfur capacity of the sorbent in a fixed-bed reactor test. The amount of sulfur absorbed by the sorbent was compared to the value assumed by stoichiometry. As the capacity expresses the sum of the sulfur reacted with zinc and iron, we call it the total sulfur capacity. Our recent study9 revealed the actual sulfidation scheme of zinc ferrite by complementary analysis of in situ X-ray diffraction and Mo¨ssbauer spectroscopy. The most significant finding was that stoichiometric zinc sulfide was produced solely at lower concentrations below the thermodynamic boundary condition. It is possible that sulfidation tests at lower concentrations enable the sulfur capacity due to zinc sulfidation to be estimated. This paper describes an attempt to divide the total sulfur capacity into its contributions from zinc and iron using a pressurizedtype thermobalance. The methodology to estimate the sulfur capacities corresponding to zinc and iron was applied to evaluate the variation in performance during multiple-cycle desulfurization. Experimental Section Specimen of Sorbent Containing Zinc FerriteSilica Composite Powder. Zinc ferrite-silica composite powder was prepared by the urea-precipitation method described in our previous work.7 The atomic ratio of the metallic elements Zn, Fe, and Si in the prepared powder was 1.0:2.0:2.0. The obtained powder was ground in an agate mortar and pestle. The com-

10.1021/ie010846r CCC: $22.00 © 2002 American Chemical Society Published on Web 05/18/2002

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Ind. Eng. Chem. Res., Vol. 41, No. 12, 2002 Table 1. Conditions for Reduction and Sulfidation Tests temp, °C pressure, MPa gas composition, vol % flow rate, cm3/min (25 °C, 0.1 MPa) H2S conca sample wt, g

Figure 1. Optical geometry and reactor schematic of the in situ XRD spectrometer.

posite powder was subjected to a series of reduction and sulfidation steps in an in situ XRD instrument (MAC Science Co., Ltd., MXP18VAHF22-SRA) to verify the reaction scheme for the zinc ferrite portion of the composite powder in a simulated coal gas environment. The desulfurization sorbent was prepared from a mixture of the composite powder and tetragonal titanium dioxide as the supporting material. The composite powder was mixed with the titanium dioxide at the specific weight ratio ZnFe2O4-SiO2/TiO2 ) 3:7. The mixture was pelletized with an extruder into a cylindrical shape of 3-mm diameter. The pellets were dried and calcined at 700 °C. The details of the preparation procedures for the composite powder and pelletized sorbent are the same as described in our previous work.6,7 The extruded pellets had various lengths and shapes. Specimens for further tests in a fixed-bed reactor and thermobalance were prepared by crushing the pellet and sieving it to 1.0-1.4 mm in diameter. In Situ XRD Analysis. The composite powder was subjected to reduction and sulfidation in an in situ XRD instrument whose details were described in a previous paper.9 The main components around the optical system of the instruments are shown schematically in Figure 1. The instrument is equipped with a high-temperature sample stage for humid, corrosive gas. Throughout our experiments, Cu KR radiation (λ ) 1.54056 Å) with an output of 12 kW (40 kV × 300 mA) was used to obtain diffraction patterns. Diffraction measurements were made for diffraction angles, 2θ, between 20 and 56° with an average scanning speed of 0.6°/min. The diffraction intensity was measured by the fixed-time method with a 2θ step of 0.04° and a count period of 4 s. Diffraction patterns of the sample were cyclically observed to identify the reaction products as a function of time. Thus, a measurement was completed every 60-min interval. Because the distinctive peaks identifying the product phases were distributed between 29 and 44°, the maximum time lag that emerged in the range of angle was only 25 min. Sample preparation on a specially manufactured quartz cell was also described in a previous work.9 The temperature applied to the sample stage was 450-550 °C. Reduction and sulfidation were performed with a simulated coal gas composition of a dry coal fed and air-blown-type gasifier as shown in Table 1. Because the reaction gas was prepared by mixing each component gas supplied by mass flow controllers, the concentration of hydrogen sulfide was easily controlled to desired value. Reduction tests were performed in the simulated coal gas without hydrogen sulfide. Sulfidation reactions were performed in the gas containing hydrogen sulfide whose concentration was adjusted between 80 and 500 ppm. This concentration range was chosen from the thermody-

in situ XRD

TGA

450-550 0.10 CO, 20; CO2, 5; H2, 8; H2O, 5; N2, balance 500

450 0.98 CO, 20; CO2, 5; H2, 8; H2O, 5; N2, balance 500

0, 80-500 ppm 0.1

0, 80-500 ppm, 1% 0.06-0.10

a Reduction performed in the reaction gas with a H S concen2 tration of 0 ppm.

Table 2. Reaction Conditions for the Desulfurization Cycle Test in a Pressurized Fixed-Bed Reactor reduction temp, °C pressure, MPa gas composition, vol % CO CO2 H2 H2O H2S O2 N2 flow rate, cm3/min (25 °C, 0.1 MPa) sample weight, g

sulfidation

regeneration

450 0.98

450 0.98

450 0.98

20.0 5.0 8.0 5.0 0.0 balance 1100

20.0 5.0 8.0 5.0 1.0 balance 3300

1.5 balance 3700

60

60

60

namic aspects and experimental restrictions described precisely in our previous work.9 Multiple Desulfurization Cycles in a Fixed-Bed Reactor. Sulfidation cycle tests of the pelletized sorbents were carried out in our apparatus with a fixedbed reactor (37.5 mm i.d.) under pressurized conditions. A continuous sulfur concentration analyzer (Yanaco Ohgi, TSA-1001) equipped with a flame photometric detector (FPD) to measure concentrations of sulfur compounds up to 2 vol % in desulfurization was used to analyze the exit gas stream. The data were used to calculate the sulfur balance of the sorbents. An oxygen analyzer (Servomex, Ltd., oxygen analyzer 540A) was used to determine the termination of the oxidation reactions. The reaction conditions are summarized in Table 2. The sieved pellets were packed into the fixedbed reactor and heated to a predetermined temperature in a stream of nitrogen gas. Sulfidation was performed by introducing a gas mixture that simulates coal gas produced in a dry coal fed, air-blown coal gasifier. Sulfidation was continued until complete breakthrough occurred. The definition of complete breakthrough is when the sulfur concentration at the reactor outlet levels off at a concentration over 90% of the inlet sulfur concentration. The total sulfur capacity of the sorbent was determined by material balance of sulfur compounds at the inlet and outlet of the sorbent bed during sulfidation. Regeneration was carried out in oxygen mixed with nitrogen, as shown in Table 2, until breakthrough of oxygen. A reduction step followed regeneration to complete the removal of sulfur. Simulated coal gas without sulfur compounds was also used in reduction (Table 2). Sulfur elution from the reactor outlet during regeneration and reduction was analyzed to evaluate the regenerability of the sorbent. Pressurized-Type Thermobalance. Thermogravimetry was carried out on a pressurized-type thermobalance designed for humid corrosive gas (ATI Chan, TG151). The obtained data were analyzed to distinguish the sulfur capacity corresponding to zinc sulfidation

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Figure 2. Reduction behavior of zinc ferrite-silica composite powder at 450 °C in coal gas.

Figure 3. Reduction behavior of zinc ferrite-silica composite powder at 550 °C in coal gas.

from the total sulfur capacity. The balance mechanism and sample furnace were used as supplied by ATI Cahn, although the humid gas supplying unit and balance elevation mechanism were designed and customized by Santeck Co., Ltd., for the specific test. The modification enabled precision measurements under pressurized gas conditions. The resolution of the balance mechanism is 1 µg at the maximum sample loading of 10 g. A sorbent specimen of typically 100 mg was loaded on the balance with a sample basket made of quartz. The sample furnace was operated at 450 or 550 °C after the gas stream was pressurized up to 0.98 MPa with nitrogen. When the weight indication was stabilized, the inert gas was changed to reduction gas. After the reduction step was continued for 1 h, the test proceeded to the sulfidation step. The gas composition for the reduction and succeeding sulfidation steps are summarized in Table 1. It was judged that termination of the sulfidation reaction occurred when the average weight change for a 10-min interval fell below 1% of the cumulative weight change. The test results were analyzed on the basis of the dry sample weight after sulfidation, because the loaded sample weight included humidity.

evidence for zincite formation was observed. At 550 °C, zinc silicate was produced during reduction, which was confirmed by the emerging (2 2 3) peak at 2θ ) 38.8°. The existence of zincite could not confirmed by the measurements, because the zincite peaks should overlap that of the zinc silicate phase. The remaining spinel phase was identified as zinc-dislocated franklinite whose peaks might superpose those of unreduced zinc ferrite. The reduction scheme identified for the composite powder was parallel to that of pure zinc ferrite, except for zinc silicate formation at 550 °C. Along with the verified reduction products, sulfidation products were considered by introducing hydrogen sulfide into the reaction gas. The sulfidation reaction scheme is essential for an analysis of thermogravimetric sulfidation data for zinc ferrite-silica composite powder. Thermodynamic calculations predicted that iron sulfide should be produced at H2S concentrations above 80 ppm for the conditions in Table 1. However, zinc sulfide can be produced at substantially lower H2S concentrations. Preceding the investigation of the sulfidation scheme of the composite powder, we examined the consistency of the reaction mechanisms between pure zinc ferrite and the double oxide in the composite powder. Because the pure zinc ferrite powder was sulfurized above 550 °C,9 we performed sulfidation of both sample at 550 °C. The sulfidation products of zinc ferrite-silica composite powder and the pure zinc ferrite are compared in Figures 4 and 5, which show diffraction patterns for sulfidation with 500 and 80 ppm of H2S. All results were obtained for a period of 20 h. The most noticeable difference between the two results is the remaining zinc ferrite for the sulfidation of pure zinc ferrite, which indicates the lower sulfidation rate of this specimen. Concerning the sulfidation products, there is a remarkable similarity between the two specimens for both zinc sulfides and iron sulfides. Although the diffraction intensity of each peak is weaker for the composite powder because of the small crystallite, all diffraction peaks of both sulfides correspond well with each other. This confirms that the sulfidation schemes of the two specimens are essentially same. Figures 6 and 7 display in situ XRD measurements of sulfidation of the composite powder in the simulated coal gas with different H2S concentrations at 450 °C. The first measurement was performed in the gas without H2S for 1 h, which ensures a reductive environment around the sample. Hydrogen sulfide was then introduced at 500 and 80 ppm. During sulfidation at

Results and Discussion Reaction Schemes of Reduction and Sulfidation of Zinc Ferrite-Silica Composite Powder. The reduction scheme of pure zinc ferrite in simulated coal gas was previously investigated9 with powder XRD and Mo¨ssbauer spectroscopy analysis. The reduction products of zinc ferrite at 550 °C were identified as zincite, zinc-dislocated franklinite, and pure magnetite. Reduction at 450 °C, however, did not evolve apparent products that were identified by in situ XRD. It was confirmed that the reduction rate at 450 °C was substantially lower than that at 550 °C. The reduction scheme of the zinc ferrite-silica composite powder was investigated based on the findings on pure zinc ferrite. Analysis with powder XRD indicated that the sample double oxide contained pure franklinite and hematite (Fe2O3). In situ X-ray measurements of the reduction of the composite powder at 450 and 550 °C are displayed in Figures 2 and 3, respectively. The silicon dioxide particles contained in the sample had diameters of about 10 nm and an amorphous texture.6 Any broadened diffraction specific to the amorphous phase was not observed during the in situ measurements. Although reduction at 450 °C indicated hematite reduction, no

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Figure 4. Comparison of diffraction patterns after 20-h sulfidation of pure zinc ferrite and zinc ferrite-silica composite powder at 550 °C in simulated coal gas environment with 500 ppm of H2S (†1, wurtzite; †2, zinc blend; †3, cubic; †4, pyrrhotite-6T).

Figure 6. Variation of X-ray diffraction pattern of pure zinc ferrite-silica composite particle during sulfidation in coal gas environment with 500 ppm H2S concentration at 450 °C (†1, wurtzite; †2, zinc blend; †3, cubic; †4, pyrrhotite-6T).

Figure 5. Comparison of diffraction patterns after 20-h sulfidation of pure zinc ferrite and zinc ferrite-silica composite powder at 550 °C in simulated coal gas environment with 80 ppm of H2S (†1, wurtzite; †2, zinc blend).

the high H2S concentration, sulfides of both zinc and iron were produced; the sulfidation products were wurtzite, zinc blende, cubic iron sulfide, and pyrrhotite. Iron sulfides were not produced at the H2S concentration of 80 ppm; zinc sulfides (wurtzite and zinc blende) were confirmed by the XRD diffraction patterns as shown in Figure 7. After 20 h of sulfidation, the spinel structure remained, which is most plausibly assigned to zinc-dislocated franklinite. The same sulfidation products were identified at the reaction temperature of 550 °C, but an acceleration of the sulfidation rate was observed. The sulfidation schemes verified by in situ XRD measurements and our previous work9 are summarized in Figure 8, which is applicable to an analysis of the weight variation data obtained with thermobalance. Sulfidation at Dual Sulfur Concentration for the Determination of Contribution to the Sulfur Capacity of Zinc and Iron. The sulfidation scheme, which was verified for the composite powder in the previous section, should be identical to the reaction schemes of desulfurization sorbents containing the same composite powder. Supporting materials in the sorbent are considered to be inert in the reaction, but a variation

Figure 7. Variation of X-ray diffraction pattern of pure zinc ferrite-silica composite particle during sulfidation in coal gas environment with an 80-ppm H2S concentration at 450 °C (†1, wurtzite; †2, zinc blend).

in the physical properties might actually affect the reaction rate. Thus, we applied the reaction schemes displayed in Figure 8 to an analysis of the thermobalance tests of the sulfidation of the sorbent. To confirm the validity of the analyses, we performed sulfidation tests of fresh sorbent. A pressurized-type thermobalance was used to determine the sulfur capacities of zinc and iron separately. We tried to distinguish the contributions of zinc and iron to sulfidation by varying the H2S concentration so that the sulfidation conditions were

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Figure 8. Verified reaction schemes for the reduction and sulfidation of pure zinc ferrite-silica composite particle in coal gas environment at 450 °C. (Oxidation schemes are estimated in refs 10 and 11.)

Figure 9. Sulfidation reaction on desulfurization sorbent containing zinc ferrite-silica composite powder at high H2S concentration and calculated sulfur capacity.

situated across the phase boundary between iron oxide and sulfide. Both zinc and iron were considered to be sulfurized at a H2S concentration of 1 vol %. The weight increase during sulfidation of fresh sorbent observed by the pressurized thermobalance is displayed in Figure 9. The total sulfur capacity was determined by analyzing the weight variation at the high H2S concentration. The total sulfur capacity of fresh sorbent was calculated as 2.48 mmol/g. This value indicates the number of moles of sulfur captured per unit weight of fresh sorbent. Because the sorbent weight varies upon reduction and sulfidation, the specific sulfur capacity was calculated in terms of the weight of unreacted fresh sorbent. The calculated value was 99.6% of the capacity determined by assuming stoichiometric sulfidation of zinc and iron. The error in determining the total sulfur capacity of fresh sorbent was reasonably small; thus, further discussion will assume this result. A sulfidation test at the lower sulfur concentration (80 ppm) was conducted to obtain an estimated zinc-related sulfur capacity of fresh sorbent. The thermobalance test results are displayed in Figure 10. The capacity was estimated as 0.72 mmol/g by assuming sulfidation products of ZnS and Fe3O4. This value was 86.2% of the value calculated by assuming complete sulfidation of zinc (0.83 mmol/ g). The difference between the estimated value and the value calculated by this method indicates the existence of unreacted zinc. Although the value obtained by our

Figure 10. Sulfidation reaction on desulfurization sorbent containing zinc ferrite-silica composite powder at low H2S concentration and calculated sulfur capacity related to zinc sulfidation.

method might be smaller than the amount of zinc that is available to be sulfurized at sufficiently high sulfur concentration, it is practically correct as the amount of reactive zinc at low sulfur concentration. Thus, the estimated sulfur capacity for zinc sulfidation is important in the practical estimation of the deep-desulfurization performance of the sorbent. This dual H2S concentration test of sulfidation was applied to evaluate the sulfur capacity of the sorbent used in multiple desulfurization cycles. We have to define the assumptions for analyzing the thermogravimetric results for such spent sorbents. The reduction of the sulfate produced in the oxidative regeneration step gives rise to sulfides; both zinc sulfate and iron sulfate are reduced to form ZnS and FeS, respectively. Although chemical forms of oxides produced in the oxidation step are not known precisely, the oxidation numbers of zinc and iron are uniquely defined to +2 and +3, respectively; ZnFe2O4, ZnO, and Fe2O3 are the expected products. Thus, the ratios of the metals to oxygen are unique for such oxides. We can assume that hematite, Fe2O3, is reduced to magnetite, Fe3O4, during reduction. In the next section, we will derive a procedure for estimating the sulfur capacities of spent sorbents by applying these assumptions and the reaction schemes. Procedure for Sulfur Capacity Analysis. Because the sorbent used in multiple-cycle desulfurization retained residual sulfur, proper data analysis of the thermobalance tests is required to calculate accurate sulfur capacities. The procedure for analyzing the thermobalance data is derived in this section to determine the sulfur capacities related to zinc and iron from sulfidation tests at dual concentrations. The zinc ferrite-silica composite powder has three metallic elements, Zn, Fe, and Si, with a 1.0:2.0:2.0 molar ratio. The weight ratio of zinc ferrite to silica in the composite powder is 2:1. Because the sorbent has a ZnFe2O4-SiO2-to-TiO2 weight ratio of 3:7, the weight fraction of zinc ferrite in the sorbent is 0.2. The remaining portion is silica and titania, whose weight fraction is 0.8. When one takes a unit weight of the sorbent, it contains 0.2 g of ZnFe2O4. It was assumed that sulfidation at 1 vol % of H2S produces stoichiometric sulfides, ZnS and FeS. This assumption allows the amounts of zinc and iron in the sorbent to be estimated from the weight of the sample that was completely sulfurized at the higher sulfur concentration. If we describe the composition of the stoichiometric sulfide as

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ZnFe2S3, its weight is expressed as

wZFS ) 0.2mZFS/mZF

(1)

Thus, the weight fraction of ZnFe2S3 in the sorbent is expressed as

χZFS ) 0.2mZFS/(0.2mZFS + 0.8mZF)

(2)

where mZFS and mZF denote the molar weights of ZnFe2S3 and ZnFe2O4, respectively. Thus, the number of moles of ZnFe2S3, nZFS, is derived by introducing the weight of sorbent after complete sulfidation, WCS, as in eq 3. The law of conservation of mass guarantees that nZFS equals the number of moles of zinc ferrite in fresh sorbent, nZF

nZFS ) nZF ) WCSχZFS/mZFS

(3)

Multiplication of nZF by the difference between the molar weights of ZnFe2S3 and ZnFe2O4 gives the expected weight increase during complete sulfidation, ∆ WMAX CS , as shown in eq 4

∆WMAX CS ) nZF(mZFS - mZF) ) nZF(3mS - 4mO)

(4)

where mS and mO denote the atomic weights of sulfur and oxygen, respectively. One can derive the estimated weight of fresh sorbent as

WZF ) WCS - ∆WMAX CS

(5)

If one assumes that the observed weight increase during sulfidation, ∆WEXP CS , is proportional to the conversion of the sulfidation reaction of both zinc and iron, then the overall conversion can be expressed as MAX XZF ) ∆WEXP CS /∆WCS

(6)

The conversion corresponds to the total sulfur capacity, FZF, which is given by

FZF ) 3XZFnZF/WZF

(7)

Because sulfidation at 80 ppm of H2S does not produce iron sulfides, the weight of complete sulfidation was estimated from the weight ratio of loaded samples as expressed in eq 8

WEST CS ) WCSMLS/MCS

(8)

where MLS and MCS express the loaded sample weights for sulfidation tests at the low and high concentrations, respectively. Then, the estimated unsulfurized sample and maximum weight increase during zinc sulfidation are expressed by eqs 9 and 10, respectively MAX W/ZF ) WEST CS - ∆WCS

(9)

where ∆WMAX CS should be recalculated by applying eqs 3 and 4 to the sample for low-concentration sulfidation

∆WMAX LS ) nZF(mZSF - mZF) ) nZF(mS - 4mO/3)

(10)

Figure 11. Sulfur capacities of desulfurization sorbent containing zinc ferrite-silica composite powder during multiple desulfurization cycles.

If one measures the actual weight increase during zinc sulfidation, the zinc-related sulfur capacity can be calculated by eq 11. The conversion gives the zincrelated sulfur capacity, FZ, as shown in eq 12 MAX XZ ) ∆WEXP LS /∆WLS

(11)

FZ ) XZnZF/W/ZF

(12)

In the next section, we adopt the above formulas to determine the sulfur capacity of sorbent that experienced multiple desulfurization cycles. Change in Sulfur Capacity Related to Zinc During Multiple Desulfurization Cycles. The total sulfur capacity of the sorbent was measured during multiple desulfurization cycles in the same apparatus as in our previous work.7 The sulfur capacity was determined by an analysis of the complete breakthrough curves obtained with the pressurized fixed-bed reactor. The reaction conditions were identical to those in Table 2, under which the thermobalance tests were conducted. The reproducibility of the sulfur capacity change was confirmed up to the 12th cycle, where we observed a relatively large decrease of the capacity. The decreasing tendency was the same as found in the 20-cycle test within experimental error. Therefore, we chose samples of spent sorbent during multiple-cycle desulfurization from the tests of up to 20 cycles. The test results for fixed-bed sulfidation are displayed by filled squares in Figure 11. Although the capacity decreased rather rapidly during the initial few cycles, the tendency slowed with repetition up to 20 cycles. The decreasing sulfur capacity is caused mainly by residual sulfur due to sulfate formation.7 The ratio of zinc sulfate to iron sulfate is not clear, because it is difficult to distinguish zinc sulfate from iron sulfate quantitatively by the usual chemical analyses. When zinc sulfate is selectively produced during regeneration, the sulfur removal performance at lower concentrations might be diminished. It is not practical to determine the zinc-related sulfur capacity with a fixed-bed reactor, because sulfidation at low H2S concentration takes a long time; the reaction time required to complete sulfidation at 80 ppm can typically be 125 times longer than that at 1 vol %. Thus, we tried to determine the change in zinc-related sulfur capacity during desulfurization cycles by applying the dual-H2S-concentration method with a pressurized thermobalance. The samples were spent sorbents taken from

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repeated desulfurization cycle tests in a fixed-bed reactor; a series of spent sorbents after the initial few cycles and after 20 cycles were subjected to the thermobalance test to determine their zinc-related sulfur capacities as well as their total sulfur capacities. The measured sulfur capacity data are plotted in Figure 11. Because each sulfidation test in thermobalance was counted as one cycle, the results are plotted as a function of the cycle of the sample plus 1. The total sulfur capacity analyzed by thermogravimetry is able to reproduce the data obtained in the fixed-bed reactor. Thermobalance tests at lower sulfur concentration (80 ppm) were performed for the same series of samples. The analyzed zincrelated sulfur capacity was subtracted from the total sulfur capacity to calculate the iron-related sulfur capacity. The calculated values are plotted in Figure 11 as filled triangles. The iron-related sulfur capacity was stable during the cycles. At the 21st cycle, the zincrelated sulfur capacity decreased to about 50% of its initial value, whereas the iron-related sulfur capacity maintained 91% of its initial value. This indicates that the decrease in the total sulfur capacity was caused by residual sulfur due to zinc sulfate. Conclusions Zinc ferrite-silica composite powder was subjected to a series of reduction and sulfidation tests performed at mainly 450 °C in a simulated coal gas environment. In situ X-ray diffraction analysis revealed that ZnS and FeS were produced at a H2S concentration of 1 vol %, whereas zinc sulfides were the sole sulfidation product at 80 ppm. The scheme for sulfidation at 450 °C was essentially the same as that of pure zinc ferrite at 550 °C. The desulfurization sorbent containing the composite powder was subjected to pressurized thermobalance to determine the sulfur capacities of zinc and iron separately by applying high and low H2S concentrations for sulfidation. The total sulfur capacity was consistent with the value obtained from the complete breakthrough curves in a fixed-bed reactor test. The sulfur capacity due to zinc sulfidation of fresh sorbent was about 86% of the expected value assuming complete sulfidation of zinc element. This method was applied to analyze the change in the zinc-related sulfur capacity of sorbent that experienced multiple desulfurization cycles up to 20 times. At the end of 20 cycles, the zinc-related sulfur capacity had fallen to approximately 50% of its initial value, whereas the sulfur capacity due to iron sulfidation maintained 91% of its initial value. This indicates

that residual sulfur during multiple desulfurization cycles is caused by the production of zinc sulfate. Literature Cited (1) Grindley, T. Sidestream zinc ferrite regeneration testing. In Proceedings of the Seventh Annual Gasification and Gas Stream Cleanup Systems Contractors Review Meeting; DOE/METC-87/ 6079; U.S. Department of Energy, U.S. Government Printing Office: Washington, DC, 1987; Vol. 2, pp 567-578. (2) Focht, G. D.; Ranade, P. V.; Harrison, D. P. High-temperature desulfurization using zinc ferrite: Reduction and sulfidation kinetics. Chem. Eng. Sci. 1988, 43, 3005-3013. (3) Focht, G. D.; Ranade, P. V.; Harrison, D. P. High-temperature desulfurization using zinc ferrite: Regeneration kinetics and multicycle testing. Chem. Eng. Sci. 1989, 44, 2919-2926. (4) Lew, S.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. Sulfidation of Zinc Titanate and Zinc Oxide Solids. Ind. Eng. Chem. Res. 1992, 31, 1890-1899. (5) Patrick, V.; Gavalas, G. R.; Flytzani-Stephanopoulos, M.; Jothimurugesan, K. High-temperature sulfidation-regeneration of CuO-Al2O3 sorbents. Ind. Eng. Chem. Res. 1989, 28, 931-940. (6) Kobayashi, M.; Shirai, H.; Nunokawa, M. Investigation on Desulfurization Performance and Pore Structure of Sorbents Containing Zinc Ferrite. Energy Fuels 1997, 11, 887-896. (7) Kobayashi, M.; Nunokawa, M.; Shirai, H. Development of regenerable desulfurization sorbent for coal gas sulfur removal below ppm level. In High-Temperature Gas Cleaning; Schmidt, E., Ga¨ng, P., Pilz, T., Dittler, A., Eds.; Univasita¨t Karlsruhe: Karlsruhe, Germany, 1996; p 618-629. (8) Kobayashi, M.; Nunokawa, M.; Shirai, H.; Watanabe, M. Activation of Desulfurization Performance of Zinc Ferrite Sorbent for Use in High-Temperature Sulfur Removal of Coal Derived Gas for Application to Molten Carbonate Fuel Cell Power Plant; CRIEPI Report EW93005; Central Research Institute of Electric Power Industry: Yokosuka, Japan, 1994. (9) Kobayashi, M.; Shirai, H.; Nunokawa, M. Elucidation of Sulfidation Mechanisms of Zinc Ferrite in a Reductive Gas Environment by in Situ X-ray Diffraction Analysis and Mo¨ssbauer Spectroscopy. Ind. Eng. Chem. Res. 2000, 39, 1934-1943. (10) Kobayashi, M.; Nunokawa, M. Development of High Performance Desulfurization Sorbent for Coal Gas Fueled MCFC Power Plant. Characteristics of Performance of Sorbents Containing Zinc and Iron during Initial Desulfurization Cycles in Pressurized Coal Gas Conditions; Report W95006; Central Research Institute of Electric Power Industry: Yokosuka, Japan, 1996. (11) Shirai, H.; Kobayashi, M.; Nunokawa, M. Development of Advanced Desulfurization Sorbent for Hot Coal Gas Cleanups Study on Optimum Condition for Regeneration of Zinc Ferrite Sorbent; Report W98018; Central Research Institute of Electric Power Industry: Yokosuka, Japan, 1999.

Received for review October 12, 2001 Revised manuscript received March 4, 2002 Accepted March 6, 2002 IE010846R